Single-layer 1970s steel panels
That matters because doors installed in the 1970s were almost always single-layer steel, no insulation, mounted on hardware sized for lighter panels than what gets sold today. When a homeowner here calls about a door that's suddenly heavy to lift by hand, it's often because the original spring was wound for a 1970s door and someone swapped in a heavier replacement panel at some point without re-rating the spring.
